Quasi-static slip on the plate boundary associated with the 2003 M8.0 Tokachi-oki and 2004 M7.1 off-Kushiro earthquakes, Japan

We analyzed small repeating earthquakes recorded over a 13-year period and GPS data recorded over an 8-month period to estimate interplate quasi-static slip associated with the 2003 Tokachi-oki earthquake (M8.0) and the 2004 off-Kushiro earthquake (M7.1). The repeating-earthquake analysis revealed that the slip rate near the source region of the Tokachi-oki earthquake was relatively low (< 5 cm/year) prior to the earthquake; however, in the last 3 years leading up to the event, a minor acceleration in slip occurred upon the deeper extension of the coseismic slip area of the earthquake. Repeating-earthquake and GPS data indicate that large amounts of afterslip occurred around the rupture area following the earthquake; the afterslip mainly propagated to the east of the coseismic slip area. We also infer that the occurrence of the 2004 off-Kushiro earthquake, located about 100 km northeast of the epicenter of the Tokachi-oki earthquake, was advanced by the afterslip associated with the Tokachi-oki earthquake.     

Initiation and development of the southern California uplift along its northern margin

Analysis of three first-order leveling lines that traverse the White Wolf fault (site of the 1952 M = 7.7 earthquake), each resurveyed nine times between 1926 and 1974, reveals probable preseismic tilting, major coseismic movements, and a spatial association between these movements and the subsequently recognized southern California uplift. In examining the vertical control record, we have both searched for evidence of systematic errors and excluded from consideration portions of the lines contaminated by subsurface fluid and gas extraction. Movements have been referred to an invariant datum based on the 1926 position of tidal BM 8 in San Pedro, corrected for subsequent eustatic sea-level change.An 8 μrad up-to-the-north preseismic tilt (6 cm/7.5 km) was apparently recorded on two adjacent line segments within 10 km of the 1952 epicenter between 1942 and 1947. It is possible, however, that this tilt was in part caused by extraction-induced subsidence at one of the six releveled benchmarks. Data also show evidence of episodic tilts that are not earthquake related. At the junction of the Garlock and San Andreas faults, for example, an ≥5 μrad up-to-the-north tilt (7.2 cm/≤16 km) took place between Lebec and Grapevine within three months during 1964.Comparison of the 1947 and 1953 surveys, which includes the coseismic interval, shows that the SW-fault end (nearest the epicenter) and the central fault reach sustained four times the uplift recorded at the NE end of the fault (+72 cm SW, +53 cm Central, +16 cm NE). A regional postseismic uplift of 4 cm extended ≥25 km to either side of the fault after the main event, from 1953 to 1956. An interval of relative quiescence followed at least through 1959, in which the elevation change did not exceed ±3 cm.The detailed pattern of aseismic uplift demonstrates that movement proceeded in space—time pulses: one half of the uplift at the SW-fault end and extending southward occurred between 1959 and 1961, one half of the uplift at the NE-fault end and extending eastward occurred between 1961 and 1965, while the central fault reach sustained successive pulses of subsidence, uplift, and collapse (−4 cm, 1953–1960; +7 cm, 1960–1965; −2 cm, 1965–1970). In addition, the number of aftershocks concentrated near the fault ends increased in the NE relative to the SW from 1952 to 1974. These observations suggest that the aseismic uplift may have migrated northeastward from 1959 to 1965 at an approximate rate of 7–16 km/yr.Evidence for a mechanical coupling between the earthquake and the subsequent aseismic uplift is equivocal. At both fault ends, the major NWbounding flexure or tilted front of the southern California uplift is spatially coincident with the coseismic flexure that preceded it. In addition, the postulated migration of vertical deformation is similar to the 1952 seismic event in which the rupture initiated at the SW end of the fault and then propagated to the NE-fault end. However, the spatial distribution of aseismic uplift, nearly identical at both fault ends and to the south and east, and near zero in the central fault reach, is distinctly different from the nonuniform and localized coseismic deformation.     

Morphotectonics and paleoseismology of the eastern end of the Bolnay fault (Mongolia)

The Bolnay (Hangayn) fault is an active shear system which generated the M = 8.2-8.5 Bolnay earthquake of 23 July 1905, one of world’s largest recorded intracontinental event. The fault follows the Mesozoic suture formed during the closure of the Mongolia-Okhotsk ocean. The Late Cenozoic faulting in the region was induced by propagation of strain from the India-Eurasia collision that had reached Mongolia at about 5 ± 3 Ma. The left-lateral strike slip almost all over the fault length is compensated in its western end by Late Quaternary reverse motion. We estimated coseismic slip associated with the event of 1905 and the previous earthquakes in the eastern fault end and checked whether vertical offset compensates the strike slip in this part as well. The 1905 coseismic slip measured from a displaced dry stream bed and pebble bars in the Hasany-Gol river valley was 6.5-7.5 m. The 13 ± 1 m left-lateral displacement of pebble bars in the same valley represents a cumulative slip of two events. Paleoseismological studies across the strike of surface ruptures reveal at least two generations of rupture in two events that postdated the deposition of sediments with a ¹⁴C age of 4689 ± 94 yr. Hypsometry of the alluvial surface in the zone of deformation shows gradual elevation increase toward the mountains, but without abrupt change across the fault. This means the absence of vertical offset and reactivation of the fault as a left-lateral strike slip. The horizontal slip in the eastern extension of the Bolnay fault is compensated rather by parallel fault-bounded pull-apart basins trending northeastward oblique to the principal fault strike. The age of their sedimentary fill suggests no older than middle Pleistocene normal faulting that compensated the Bolnay strike slip.     

Trenching exposures of the surface rupture of 2008 Mw 7.9 Wenchuan earthquake,China: Implications for coseismic deformation and paleoseismology along the Central Longmen Shan thrust fault

The May 12, 2008, Mw 7.9 Wenchuan earthquake was induced by failure of two of the major faults of the Longmen Shan thrust fault zone along the eastern margin of Tibet Plateau. Our study focused on trenches across the Yingxiu–Bichuan fault, the central fault in the Longmen Shan belt that has a coseismic surface break of more than 200 km long. Trenching excavation across the 2008 earthquake rupture on three representative sites reveals the styles and amounts of the deformation and paleoseismicity along the Longmen Shan fault. Styles of coseismic deformation along the 2008 earthquake rupture at these three sites represent three models of deformation along a thrust fault. Two of the three trench exposures reveal one pre-2008 earthquake event, which is coincident with the pre-existing scarps. Based on the observation of exposed stratigraphy and structures in the trenches and the geomorphic expressions on ground surface, we interpret the 2008 earthquake as a characteristic earthquake along this fault. The interval of reoccurrence of large earthquake events on the Central Longmen Shan fault (the Yingxiu–Beichuan fault) can be inferred to be about 11,000 years according to ¹⁴C and OSL dating. The amounts of the vertical displacement and shortening across the surface rupture during the 2008 earthquake are determined to be 1.0–2.8 m and 0.15–1.32 m, respectively. The shortening rate and uplift rate are then estimated to be 0.09–0.12 mm/yr and 0.18–0.2 mm/yr, respectively. It is indicated that the deformation is absorbed mainly not by shortening, but by uplift along the rupture during the 2008 earthquake.     

Interaction between seismic and volcanic deformation: a possible interpretation of ground deformation observed at Vulcano Island (1976–84)

Levelling measurements carried by Osservatorio Vesuviano over Vulcano Island (North Sicily) since June 1976 show a complex history of small but significant vertical movements which took place after the ‘Patti earthquake’ of 15/4/1978. Three phases can be identified in the time history of ground deformation: (I) co-seismic subsidence of the northern part of the Island with respect to the southern part, followed by (II) transient inflation of the central region surrounding ‘La Fossa’ volcano, and finally (III) deflation and subsidence to stationary values greater than phase I. Large variations in temperature and gas content of magmatic origin were also detected in fumaroles along the crater rim of ‘La Fossa’. The coincidence in time of the earthquake with the start of the deformation history, the proximity of the event to Vulcano Island and the favourable mechanism (right lateral) and orientation (NNW) of the fault strike, suggest that the earthquake triggered the observed deformation. Among the plausible models, one seems particularly able to reproduce the amplitude and time evolution of the geodetic data, in a manner consistent with the geochemical observations. According to this model the earthquake, while generating a compressive stress field in the quadrant of Vulcano Island, induced at first a relative coseismic subsidence of the northern part of the island, accompanied by a large increase of the mean stress within a magma chamber at ～ 6.5 km depth. Volatile fluids were then released from the top of the magma chamber and migrated towards the surface following the enhanced pressure gradient. As a result fluid pressure diffusion around the magma chamber induced a local transient uplift of ground that vanished after exhaustion of the overpressure at the source, and finally left a permanent dislocation deformation, possibly amplified by the post-seismic relaxation of the earthquake deviatoric stress. This interpretation is qualitatively consistent with the one suggested by Ferri et al. (1988).     

2008年汶川地震同震滑移特征、最大滑移量及构造意义

2008年汶川地震（M_s8.0）形成了迄今为止空间上分布最为复杂、长度最大的逆冲型同震地表破裂带。沿约275km长的地表破裂带的同震滑移及其最大滑移量的确定,对认识和理解汶川地震地表破裂过程及其变形机制具有重要意义。我们沿地表破裂带进行了详细的滑移特征考察及其同震位移测量,发现沿映秀-北川破裂带分布南北两个滑移峰值区段,南段以深溪沟-虹口破裂段为中心,以逆冲为主伴随右旋走滑运动为特征,最大垂直位移量为6.0～6.7m,北段以北川破裂段为中心,以右旋走滑为主伴随逆冲运动为特征,最大垂直位移量为11～12m,南北两滑移峰值区段所代表的两次地表破裂事件与地震波数据反演结果一致。通过对北川段破裂带的精细地形剖面测量,以及地震前后对比,在北川县曲山镇沙坝村一组获得该破裂段的最大右旋水平位移为12～15m,最大垂直位移为11～12m,这是目前世界上一次地震产生的最大同震垂直位移,最大斜向滑移量为14～17m,为整个汶川地震地表破裂最大滑移量,是汶川地震的宏观震中。北川破裂段高角度的地震断裂、逆冲断裂面的倒转作用以及具最大滑移量的强烈变形作用是北川县城遭受到最强的地表破坏和地质灾害的主要原因。具有走滑量和逆冲量近一致（走滑水平位移/逆冲垂直位移比值为1）的斜向逆冲作用可能是山脉快速隆升的重要机制。     

Palaeoseismology in steep terrain: The Big Bend of the San Andreas fault, Transverse Ranges, California

Transpressional tectonics are typically associated with restraining bends on major active strike-slip faults, resulting in uplift and steep terrain. This produces dynamic erosional and depositional conditions and difficulties for established lines of palaeoseismological investigation. Consequently, in these areas data are lacking to determine tectonic behaviour and future hazard potential along these important fault segments. The Big Bend of the San Andreas fault in the Transverse Ranges of southern California exemplifies these problems. However, landslides, probably seismically triggered, are widespread in the rugged terrain of the Big Bend. Fluvial reworking of these deposits rapidly produces geomorphic planes and lines that are markers for subsequent fault slip. The most useful are offset and abandoned stream channels, for these are relatively high precision markers for identifying individual faulting events. Palaeoseismological studies from the central Big Bend, involving ¹⁴C ages of charcoal fragments from trench exposures, illustrate these points and indicate that the past three faulting events, including the great 1857 earthquake, were relatively similar in scale, each producing offsets of about 7–7.5 m. The mean recurrence interval is 140–220 years. The pre-1857 event here may be the 1812 event documented south of the Big Bend or an event which took place probably between 1630 and 1690. Ground breakage in both events extended south of the Big Bend, unlike the 1857 event where rupture was skewed to the north. The preceding faulting event ruptured both to the north and south of the Big Bend and probably occurred between 1465 and 1495. All these events centred on the Big Bend and may be typical for this fault segment, suggesting that models of uniform long-term slip rates may not be applicable to the south-central San Andreas. A slip-rate estimate of 34–51 mm a^{− 1} for the central Big Bend, although uncertain, may also imply higher slip in the Big Bend and highlights difficulties in correlating slip-rates between sites with different tectonic settings. Slip rates on the San Andreas may increase within the broad compressional tectonics zone of the Big Bend, compared to the north and south where the plate boundary is a relatively linear and sub-parallel series of dominantly strike-slip faults. Partitioning slip within the Big Bend is inherently uncertain and insufficient suitably comparable data are available to sustain a uniform slip model, although such models are a common working assumption.     

The Altun Fault: Its Geometry, Nature and Mode of Growth

The Altun (or Altyn Tagh) fault displays a geometry of overlapping of linear and arcuate segments and shows strong inhomogeneity in time and space. It is a gigantic fault system with complex mechanical behaviours including thrusting, sinistral strike slip and normal slip. The strike slip and normal slip mainly occurred in the Cretaceous-Cenozoic and Plio-Quaternary respectively, whereas the thrusting was a deformation event that has played a dominant role since the late Palaeozoic (for a duration of about 305 Ma). The formation of the Altun fault was related to strong inhomogeneous deformation of the massifs on its two sides (in the hinterland of the Altun Mountains contractional deformation predominated and in the Qilian massif thrust propagation was dominant). The fault experienced a dynamic process of successive break-up and connection of its segments and gradual propagation, which was synchronous with the development of an overstep thrust sequence in the Qilian massif and the uplift of the Qinghai-T     

基于形变观测分析中国台湾东部花东纵谷断层运动特征

花东纵谷断层是中国台湾动力作用和地壳运动变形最强烈的断层之一,其断层运动特征和强震危险程度一直备受学者的关注。文中分别以同震地表位移、1992-1999年震间形变数据为约束,反演2003年成功MW 6.8地震同震位错分布和花东纵谷断层震间运动特征。结果表明：花东纵谷断层北段处于强闭锁状态（闭锁率高达0.9）,闭锁深度深（约27 km）;南段闭锁程度较弱（闭锁率约0.5）,闭锁深度较浅（约12 km）;中段闭锁程度与闭锁深度介于南北段之间。另一方面,2003年成功MW 6.8地震微观震中位于震间无震滑移区与闭锁区的过渡带附近。依据同震位错、震间断层运动反演结果,以及历史强震破裂分布特征,分析认为,花东纵谷断层南北段运动方式存在差异性,北段主要以强震形式运动,南段以蠕滑和地震两种形式运动。自1951年花莲-台东ML 7.3地震序列后,花东纵谷断层南段、中段和北段至2016年所累积的矩能量分别等价MW 6.4、MW 7.0、MW 7.4地震;若发生级联破裂,整个断层至2016年所累积的矩能量等价MW 7.5地震。     

阿尔金断裂：几何学、性质和生长方式

中央造山带范围内的诸多新元古代晚期-早古生长代蛇绿岩带可视为一个系统--昆祁秦缝合系。大致可分西、中、东3段。西段包括库地-苏巴什和麻扎-康西瓦2条缝合带及西昆中微地块；中段包括阿尔金、北祁连、柴达木北缘、祁漫塔格-乌妥及东昆仑南缘5条缝合带和中-南祁连、柴达木、阿牙克库木及玛沁4个较大的微地块及诸多较小的微地块；东段为商丹缝合带和勉略缝合带及其所夹持的秦中-大别微地块。昆祁秦缝合是Rodinia超大陆经历了早-中震旦世（约780-600Ma）初期裂解阶段，晚震旦世-奥陶纪（约600-440Ma）昆祁秦多     

Co‐seismic Faults and Geological Hazards and Incidence of Active Fault of Wenchuan Ms 8.0 Earthquake,Sichuan, China

Abstract: There are two co-seismic faults which developed when the Wenchuan earthquake happened. One occurred along the active fault zone in the central Longmen Mts. and the other in the front of Longmen Mts. The length of which is more than 270 km and about 80 km respectively. The co-seismic fault shows a reverse flexure belt with strike of N45°–60°E in the ground, which caused uplift at its northwest side and subsidence at the southeast. The fault face dips to the northwest with a dip angle ranging from 50° to 60°. The vertical offset of the co-seismic fault ranges 2.5–3.0 m along the Yingxiu-Beichuan co-seismic fault, and 1.5–1.1 m along the Doujiangyan-Hanwang fault. Movement of the co-seismic fault presents obvious segmented features along the active fault zone in central Longmen Mts. For instance, in the section from Yingxiu to Leigu town, thrust without evident slip occurred; while from Beichuan to Qingchuan, thrust and dextral strike-slip take place. Main movement along the front Longmen Mts. shows thrust without slip and segmented features. The area of earthquake intensity more than IX degree and the distribution of secondary geological hazards occurred along the hanging wall of co-seismic faults, and were consistent with the area of aftershock, and its width is less than 40km from co-seismic faults in the hanging wall. The secondary geological hazards, collapses, landslides, debris flows et al., concentrated in the hanging wall of co-seismic fault within 0–20 km from co-seismic fault.     

Retrieving three-dimensional coseismic displacements of the 2008 Gaize,Tibet earthquake from multi-path interferometric phase analysis

In this paper, synthetic aperture radar (SAR) data from ENVISAT ASAR ascending, descending and ALOS PALSAR ascending orbits are collected to investigate the coseismic displacements of the Mw 6.4 earthquake occurred in Gaize, Tibet on January 9, 2008 and the Mw 5.9 aftershock on January 16, 2008. Two interferometric phase analysis techniques, i.e., D-InSAR and multi-aperture InSAR, are employed to process the SAR data, with which the displacement measurements along three different line-of-sight (LOS) and three different azimuth directions are retrieved, respectively. Complete three-dimensional (3-D) coseismic displacement fields caused by the earthquake are then resolved by integrating the obtained LOS and azimuth displacement measurements with a weighted least squares adjustment, whose distributions are conformed to the two north-northeast trending northwest-dipping normal faults detected in previous studies. Ground subsidence and uplift are observed in the hanging wall and footwall of the main fault, respectively, and the subsidence reaches its maximum in the hanging wall of the second fault as a superimposed result of the Gaize earthquake and its aftershock. Anti-symmetric horizontal movements are also detected during the seismic events, which move inward in the focal region, but outward at the marginal. The left-lateral motions near the main fault indicate a small striking slip component caused by the Gaize earthquake. Finally, we discuss the potential of applying the derived spatially continuous 3-D displacement fields to determine the high-resolution 3-D strain fields of the Gaize earthquake, which provide important knowledge for assessing the source mechanism.     

The Helice Fault?

The site of the ancient Greek city of Helice, which was destroyed by an earthquake in 373 bc , has never been found. We propose that it was submerged as the result of seismic slip on a normal fault traceable SE of Egion and that the fault was reactivated in 1881, when Schmidt reported a fresh scarp 2 m high and 13 km in length at the same locality. Earlier movements on the fault are inferred from Lithophaga borings on coastal Sectors of the footwall which yield ¹⁴C ages representing over 6.5 m of uplift in the last 4880 years.     

两次于田M_S7.3地震间应力触发作用及2014年于田地震的发生对周缘断层的影响

基于青藏高原及邻区的三维粘弹性有限元模型,讨论2008年于田MS7.3级地震与2014年于田MS7.3级地震之间的关系,并研究2014年于田MS7.3级地震的发生造成周围断层的库仑破裂应力变化。初步结果表明:1)2008年于田MS7.3级地震在2014年于田MS7.3级地震震中滑动方向上产生的库仑破裂应力变化高于地震触发的阈值0.01 MPa,存在明显的触发作用。在视摩擦系数分别取0.4和0.6时,震源区同震库仑破裂应力变化为0.0167 MPa和0.0170 MPa;而考虑粘弹性松弛作用时产生的库仑应力增加量分别为0.0187 MPa和0.0194 MPa。结合断裂带构造应力年累计速率的结果,2008年于田地震的发生造成2014年于田地震提前21.4~24.9 a;2)在较短的时间尺度内,对于距离相近的两次地震之间,同震产生的应力变化远大于粘弹性松弛效应产生的变化;3)2014年于田MS7.3级地震的发生造成阿尔金断裂中北段、玛尼—玉树断裂中段、东昆仑断裂西段、柴达木北缘断裂东段、西秦岭北缘断裂西段等不同程度的加载效应,地震危险性有所增强。其中阿尔金断裂中段库仑应力增加最为明显,最大达2.8×10–3 MPa;玛尼—玉树断裂中段次之,应力增加量最大达5.6×10–4 MPa;东昆仑断裂西段应力增加量最大达4.75×10–4 MPa。而玛尼—玉树断裂西段库仑破裂应力最大卸载量达3.6×10–3 MPa。     

Imaging the ramp–décollement geometry of the Chelungpu fault using coseismic GPS displacements from the 1999 Chi-Chi, Taiwan earthquake

We use coseismic GPS data from the 1999 Chi-Chi, Taiwan earthquake to estimate the subsurface shape of the Chelungpu fault that ruptured during the earthquake. Studies prior to the earthquake suggest a ramp–décollement geometry for the Chelungpu fault, yet many finite source inversions using GPS and seismic data assume slip occurred on the down-dip extension of the Chelungpu ramp, rather than on a sub-horizontal décollement. We test whether slip occurred on the décollement or the down-dip extension of the ramp using well-established methods of inverting GPS data for geometry and slip on faults represented as elastic dislocations. We find that a significant portion of the coseismic slip did indeed occur on a sub-horizontal décollement located at 8 km depth. The slip on the décollement contributes 21% of the total modeled moment release. We estimate the fault geometry assuming several different models for the distribution of elastic properties in the earth: homogeneous, layered, and layered with lateral material contrast across the fault. It is shown, however, that heterogeneity has little influence on our estimated fault geometry. We also investigate several competing interpretations of deformation within the E/W trending rupture zone at the northern end of the 1999 ground ruptures. We demonstrate that the GPS data require a 22- to 35-km-long lateral ramp at the northern end, contradicting other investigations that propose deformation is concentrated within 10 km of the Chelungpu fault. Lastly, we propose a simple tectonic model for the development of the lateral ramp.     

Slip distribution and source parameters of the 20 July 2017 Bodrum-Kos earthquake (Mw6.6) from GPS observations

Greek-Turkish boundary near the cities Kos and Bodrum has been shaken on July 20, 2017 by a Mw6.6 earthquake. The mainshock is located offshore and did not generate an on-land surface rupture. Analyzing pre- and post-earthquake continuous/survey-type static GPS observations, we investigated co-seismic surface displacements at 20 sites to characterize source parameters and slip-distribution of the mainshock. Fault plane solutions as well as co-seismic slip distribution have been acquired through the inversion of co-seismic GPS displacements modeling the event as elastic dislocations in a half space. Fault plane solution shows a southward dipping normal-type fault segment extending a depth down to ~12 km, which remains within the brittle upper crust. Results from the distributed slip inversion show that the mainshock activated a ~65 km fault section, which has three high slip patches, namely western, central and eastern patches, where the coseismic slips reach up to 13, 26, and 5 cm, respectively. This slip pattern indicates that the pre-earthquake coupling, which is storing the slip deficit, occurred on these three patches.     

Coulomb stress triggering of earthquakes along the Atalanti Fault, central Greece: Two April 1894 M6+ events and stress change patterns

Two M6+ events occurred 15–20 km apart in central Greece on April 20 and April 27, 1894. We identify the April 27, 1894 rupture (2nd in the sequence) with the Atalanti segment of the Atalanti Fault Zone because of unequivocal surface rupturing evidence reported by Skouphos [Skouphos, T., 1894. Die swei grossen Erdbeben in Lokris am 8/20 und 15/27 April 1894. Zeitschrift Ges. Erdkunde zu Berlin, vol. 24, pp. 409–474]. Coulomb stress transfer analysis and macroseismic evidence suggest that the April 20, 1894 event (1st in the sequence) may be associated with the Martinon segment of the same fault zone. Our stress modelling suggests that this segment may have ruptured in an M = 6.4 event producing a 15-km long rupture which transferred 1.14 bar in the epicentral area of the April 27th, 1894 event, thus triggering the second M = 6.6 earthquake along the Atalanti segment and producing a 19-km long rupture. We also examined three alternative fault sources for the first event; however, all these produce smaller stress stresses for triggering the second event. The proposed slip model for the second earthquake is capable of producing coastal subsidence of the order of centimetres to decimetres, which fits the geological data. The 1894 earthquake sequence was followed by a difference in the timing of subsequent M > 5 events in each of the “relaxed” areas (stress shadows; a negative change in Coulomb failure stress > − 0.6 bar), which terminated between 22–37 years (north) and 80 years (south).     

Drainage diversions as evidence of propagating active faults: example of the El Asnam and Thenia faults, Algeria

Digital elevation models and aerial photographs provide evidence of past drainage diversions associated with known and inferred active faults in Northern Algeria. The El Asnam anticline is a fault-bend fold that is growing in response to displacement on the historically active El Asnam fault. The diversion of the Fodda River and a sequence of uplifted palaeovalleys across the anticline formed in response to the active southwestward propagation of the Sidi Ada–El Ardja segment of the El Asnam anticline. Deformation associated with the 1980 El Asnam earthquake (Ms 7.3) and its effect on drainage demonstrate the occurrence of coseismic uplift of the El Asnam anticline associated with slip on the underlying thrust fault. Similar geomorphic relations are observed between the Isser River and the Thenia fault. Diversions of the Isser River and evidence for Quaternary uplift indicate that the Thenia fault, which is close to Algiers, is tectonically active and should be considered to be a source of potentially damaging earthquakes.     

Morphological and microtectonic analysis of Quaternary deformation from Puná and Santa Clara Islands, Gulf of Guayaquil, Ecuador (South America)

This paper presents the neotectonic study of Santa Clara and Puná Islands sited in the Gulf of Guayaquil eastern part. Both islands are located on the south-western segment of the fault zone bounding to the east the North Andean Block. Fault motion and morphostructural analysis were carried out from Pleistocene age terrain. A two step deformation characterises the South Puná tectonics. The first step involves the Zambapala Cordillera uplift that post-dates Pleistocene sediments and pre-dates a marine terrace correlated with the M.I.S. 11 or 13 (440–550 ka). The second step is the formation of a pull-apart that shows evidence of 2.9 km dextral offset since the M.I.S. 11 or 13, giving an offset mean rate of 5.3 to 6.6 mm/yr. This rate is higher than the one calculated on the Pallatanga Fault northeast of the study area, in the Western Andean Cordillera, suggesting that deformation is split in different fault segments from the Gulf of Guayaquil to the continent. The Zambapala Cordillera uplift and transpression deformation requires a compressive event that may have been induced by the subduction process during the early Pleistocene.